Terrorists could acquire nuclear weapons by using weapon-usable nuclear material that was stolen or otherwise diverted from legitimate authorities. Multiple well-documented seizures suggest the existence of a black market that draws on an unknown stock of weapon-usable nuclear material that is not under the control of authorities. We estimate the total amount of uncontrolled material based on publicly reported seizures and several different statistical methods and models. We estimate that 90 to 250 kilograms—sufficient for up to ten nuclear weapons—remain outside the control of legitimate authorities. While this estimate is subject to large uncertainties and potential bias, governments may have additional information about nuclear material seizures that could be used to improve estimates.